The sterilization of various bacteria, viruses and other microorganisms and the growth suppression of plants and algae is essential for many medical treatments, preventive hygiene, food processing and preservation, and industrial and domestic applications. Known sterilization techniques employ heat, electricity, microwaves, radiation, chemicals, toxicants, antibiotics, cold, ultraviolet and far-infrared rays, and high magnetic fields. Such sterilization techniques, however, generally require large equipment and/or facilities, and may damage the object being sterilized or become toxic to the patient undergoing treatment.
It has been proposed to employ photocatalytic materials (also referred to as photosemiconductors), such as titania (TiO.sub.2), as membranes and filters to eliminate organic pollutants and other impurities from a fluid source such as water. UV irradiation of the photocatalytic material in water at an appropriate wavelength (less than 400 nm for titania) generates excess electrons in the conduction band leaving behind positively charged holes in the valence band. It is suggested in U.S. Pat. Nos. 5,126,111 and 5,118,422 that the oxidizing holes generated in the valence band react with adsorbed H.sub.2 O or surface OH.sup.- groups to form extremely reactive OH radicals. The electrons react with molecular oxygen to form superoxide ions (O.sub.2 --) which subsequently leads to the formation of additional OH radicals. The OH radicals are considered essential to purification of the contaminated fluid.
The apparent choice of the prior art has been to form such photoreactive ceramic membranes in a sol-gel process which involves precipitation of a sol containing titania in solution (typically water or alcohol), removal of the solvent to create a semisolid gel and then calcination of the gel until a rigid porous membrane is formed. The ceramic membrane may be self-supporting or coated onto a mesh substrate prior to gellation. U.S. Pat. Nos. 5,227,342, 5,192,452, 5,035,784 and 4,892,712 are representative of conventional sol-gel formed photosemiconductor membranes. The sol-gel process suffers from several deficiencies. It is complicated, time consuming and expensive. The sol-gel process is not believed to be indicated for use with substrates made of paper, low melting point plastic mesh or other heat susceptible or liquid absorbable material which may be weakened, damaged or destroyed by the soaking, evaporating and firing steps required in sol-gel formation. The fired sol-gel membranes are likely to be brittle and susceptible to flaking and other deterioration of the photosemiconductive material, particularly after prolonged contact with the fluid being purified. Further, it would appear to be difficult to provide a porous membrane, necessary for filtration, having the required mechanical strength. Although sintering will stabilize the ceramic, the downside is a reduction in porosity due to grain growth.
An alternative and more advantageous approach for forming photocatalytic materials is generally described by Sakurada in Japanese patent nos. H3-60911, H3-65430 and H3-8448. Low temperature melt injection (LMI) is employed to create a thin, porous and flexible film of a particulate photosemiconductor on a heat susceptible substrate such as cloth, paper or plastic, previously electroless plated with a conductive metal such as nickel, without thermally damaging the substrate. The excited conduction band electrons, upon irradiation of the photosemiconductor film, migrate to the conductive metal where they act as a reducing agent. The positively charged holes in the photosemiconductor operate as an oxidizing agent. Organic materials, such as bacteria and viruses, which come into contact with the cloth are eliminated. When the film is immersed in water, the active species generated on the surface of the photosemiconductor particles and the electroless silver plated cloth migrate in the water to attack spatially separated microorganisms which are not within the intimate reach of the reactive surfaces. The photosemiconductor film and electroless plated substrate provide an improved sterilizing efficiency as compared to the photosemiconductor film alone. The metal conductor is believed to act as an electron attractor, delaying recombination of the conduction band electrons and valence band holes which are essential to destruction of the offending substances. Alternatively, the Sakurada references proposed coating the photosemiconductor particles with platinum or ruthenium oxide and then melt spraying the particles onto a mesh substrate.
The LMI technique involves entraining the fine photosemiconductor particles in a fast moving combustible carrier gas, igniting the gas to form a flame which melts the fine particles and carries the molten mist at high speed onto a cloth or other heat susceptible substrate. A cold accelerating gas injected from side ports jettisons the molten particles toward the substrate. The fine molten particles penetrate into and coat the mesh fibers of the substrate and then solidify very quickly, forming a binderless, flexible and porous photocatalytic film. Despite the fact that the ultrafine particles are melted at temperatures in excess of 1000.degree. C., and sometimes as high as 3000.degree. C., a heat susceptible substrate such as cloth or paper does not become damaged. LMI is simpler, faster and less expensive than the sol-gel process described in the aforementioned prior art, and results in a flexible, yet durable, photocatalytic film which can be coated onto thermally frail materials without adverse consequences. The high porosity of the LMI film enhances catalysis which is surface area dependent.
Investigation of the LMI process has revealed that the ability to adsorb microorganisms from a fluid to allow sufficient time for sterilization of the antimicrobial film would be beneficial. It also has been discovered by applicants that the reactive ability of the anti-microbial film can be enhanced by ensuring that an aqueous environment is maintained around the sterilizing material. The versatility and efficiency of the prior art LMI anti-microbial films also were believed to be enhanceable. It was not apparent from the prior art how to accomplish these newly discovered goals, which are achieved by the present invention as delineated below.